Fragmentation of alkoxide ions following collisional activation. An energy-resolved study

Abstract

. Can. J. Chem. 66, 2947Chem. 66, (1988.The collisionally activated dissociation reactions of the C2 to C5 alkoxide ions have been studied for collisons occurring at 8 keV kinetic energy and also over the range 5 to 100 eV kinetic energy. The alkoxide ions fragment by 1,2-elimination of H2 andlor an alkane. Thus, primary alkoxide ions fragment by elimination of H, only, secondary alkoxide ions show elimination of Hz and alkane molecules, while tertiary alkoxide ions show elimination of alkanes only. In alkane elimination, loss of CH4 is much more facilie than loss of larger alkanes. For secondary alkoxide ions, where more than one elimination reaction occurs, the energy dependence of fragmentation has been explored over the collision energy range 5 to 100 eV. The results are interpreted in terms of a step-wise mechanism involving formation of an anion-carbonyl compound ion-dipole complex, followed by proton abstraction by the H-or alkyl anion leading to the final products. The relative importance of the reaction channels is determined by the relative stabilities of these ion-dipole complexes. ROGER S. MERCER et ALEX G. HARRISON. Can. J. Chem. 66,2947 (1988.On a CtudiCe les reactions de dissociation des ions alcoolates C2 a CS activkes par des collisions se produisant i une Cnergie cinktique de 8 keV ainsi qu'i des energies cinetiques allant de 5 a 100 eV. Les ions alcoolates se fragmentent par une elimination-l,2 de Hz eVou d'alcane. Ainsi, les ions alcoolates primaires se fragmentent uniquement par une elimination de Hz; par ailleurs, les ions alcoolates secondaires presentent de l'elimination de H2 ainsi que celle de molecules d'alcanes alors que les ions alcoolates tertiaires ne presentent que de l'elimination d'alcanes. Lors de l'elimination d'alcanes, la perte de CH4 est beaucoup plus facile que celle des alcanes plus volumineux. Dans le cas des alcoolates secondaires oh il se produit plus d'une elimination, on a CvaluC la relation qui existe entre I'energie de collision (de 5 100 eV) et le type de fragmentation. On interprete les resultats en fonction d'un mecanisme par Ctape impliquant la formation d'un complexe ion-dip6le d'un compose anioncarbonyle, suivie par l'enlevernent d'un proton par un H-ou par un anion alkyle, conduisant aux produits. On a determine l'importance relative des diverses voies rtactionnelles en se basant sur les stabilites relatives de ces complexes ion-dip6les.[Traduit par la revue] Introduction Studies of the fragmentation of gaseous ions have long been invaluable sources of information concerning the structures and chemistry of such species (1, 2). This information is even more useful if the internal energy of the fragmenting ions is known and can be varied in a controlled fashion; such energy-resolved studies permit the establishment of break-down graphs expressing the variation of fragment ion yields with internal energy. Energy-resolved data for odd-electron molecular cations can be obtained with high precision by photoelectron/photoion coincidence (PEPICO) experiments (3-5)...

Correlations of ion structure with multiple fragmentation pathways arising from collision‐induced dissociations of selected α‐hydroxycarboxylic acid anions

“…Overall, the fragmentation pathways deduced for lactate (Fig. ) are consistent with the stepwise dissociation process resulting from previous experimental and theoretical mechanistic studies of the gas‐phase, unimolecular decomposition of aliphatic alkoxide ions. Furthermore, energies computed for the formation of the product ions are below the maximum energy transfers to ions of modest masses on collision with argon atoms…”

“…As a simple initial step in the analysis of a product‐ion spectrum, the recognition of complementary pairs of product ions would pinpoint the site of bond cleavage and indicate proton transfer within an ion‐neutral complex as a dissociative mechanism. Further examples of complementary pairs of product anions are seen upon CID of alkoxide ions …”

Characterization of α‐ and γ‐glutamyl dipeptides by negative ion collision‐induced dissociation

“…The mechanistic information obtained from these studies has been summarized by Bowie and co‐workers 11, 12. A recent study13 of the low‐energy CID of deprotonated peptides containing H or alkyl α‐groups indicated that more extensive and sequence‐specific fragmentation was observed in low‐energy CID than in high‐energy CID for deprotonated peptides; this result is consistent with earlier studies14–17 of non‐peptide systems which showed more extensive and structure‐specific fragmentation in low‐energy CID of negative ions. Recently, Bowie and co‐workers have carried out extensive studies of the low‐energy CID of larger deprotonated peptides 18–24.…”

An ab Initio Study of the [C2H5O-] Potential Energy Surface and the Fragmentation Pathways of the Ethoxide Anion

“…The gas-phase fragmentations of simple and more complex alkoxides have been well studied by both collision activated dissociation (CAD) − and infrared multiple photon (IRMP) − induced elimination techniques. Hayes et al , investigated extensively the CAD of some simple alkoxides, e.g., ethoxide, propoxide, as well as butoxide, and suggested that the loss of neutral molecular hydrogen (H 2 ) from ethoxide and methane (CH 4 ) from butoxide proceed via a stepwise 1,2-elimination mechanism as illustrated below: Additionally, Raftery and co-workers 3 found that the loss of H 2 from complex alkoxide ions such as Ph(CH 2 ) n O - , where n = 1−5, may occur by similar 1,2-elimination mechanisms.…”